1. Field of the Invention
The present invention relates to a method of fabricating semiconductor devices such as thin-film transistors (TFTs) and, more particularly, to a method of fabricating semiconductor devices using a silicon film which is crystallized by the catalytic action of a metal element.
2. Description of the Related Art
In recent years, a configuration using TFTs as liquid crystal devices has attracted attention. This is known as an active matrix liquid crystal display and has millions of pixels arranged in rows and columns. TFTs are connected with each pixel. This liquid crystal display is characterized in that the manner in which electric charge is stored in pixels and transmitted out of them is controlled by these TFTs. This active matrix liquid crystal display is capable of providing a display with high information content and at a high speed. Consequently, the active matrix liquid crystal display is used in portable wordprocessors and computer displays.
Although amorphous silicon film is conveniently used as a silicon film incorporated in TFTs, the electrical characteristics of the amorphous silicon film are much poorer than those of single-crystal semiconductor used in a semiconductor integrated circuit. Therefore, TFTs using the amorphous silicon film can be used only in limited applications such as switching devices in active matrix circuits.
In order to improve the characteristics of a TFT, a silicon film having crystallinity may be used. Besides single-crystal silicon, polycrystalline silicon film and crystallite silicon film are known as silicon films having crystallinity. In order to obtain a silicon film having such crystallinity, an amorphous silicon film is grown and then heated by thermal annealing to crystallize the amorphous film. This method is known as solid phase epitaxy, because the crystal state is changed from amorphous state to crystalline state while the solid phase is maintained.
Generally, liquid crystal displays are required to use substrates having transparency and so limitations are imposed on the substrate material. Generally, a material which satisfies the various requirements, i.e., it has transparency, is cheap, and provides a large area, is only glass.
Where silicon is grown by solid phase epitaxy, the heating temperature is above 600.degree. C. and the heating time is more than 10 hours. Corning 7059 glass which is widely accepted into general use has a strain point of 593.degree. C. Where increases in area of substrates are taken into account, it is difficult to perform thermal annealing above 600.degree. C.
Process Leading to the Invention
In view of these problems, we have conducted researches. We have found that if a trace amount of a metal element is added to an amorphous silicon film, crystallization of silicon is promoted by the catalytic action of the metal element, and that the crystallization temperature can be lowered and the crystallization time can be shortened. More specifically, we have discovered that silicon can be crystallized by performing a heat-treatment at 550.degree. C. for about 4 hours. Therefore, TFTs using a crystalline silicon film can be fabricated on a glass substrate.
One or more elements selected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au can be appropriately selected as the aforementioned metal element producing catalytic action. Among them, nickel (Ni) produces especially conspicuous crystallization effect.
Methods for introducing metal elements into amorphous silicon film include (i) a method consisting of bringing a coating, particles, clusters, or the like including a metal element into intimate contact with an amorphous silicon film, (ii) a method of consisting of applying an aqueous solution containing a metal element, and (iii) an ion implantation method. An amorphous silicon film in which a metal element has been introduced is crystallized by heating the film at a temperature of 450-580.degree. C. for 4 to 8 hours.
As a result of our research, we have elucidated the crystallization process of silicon where a metal element is added. That is, heating causes amorphous silicon to react with the metal element, thus forming silicide. Then, silicon is heteroepitaxially grown on the surface of the silicide which acts as embryos. The dimensions of embryos of silicide depend on thermal factors, the thickness of the amorphous silicon, and other factors. Where nickel is used as a metal element, the dimensions are on the order of 500-2000 .ANG.. Therefore, obtained silicon crystals are pillar-shaped crystals having widths comparable to those of the embryos. The crystals grow from regions in which the metal element has been introduced toward the surroundings. Consequently, the crystal growth can be controlled by controlling the regions in which the metal element is introduced and their shapes.
FIGS. 6(A)-6(D) illustrate the prior art process of crystallization of silicon, utilizing the catalytic action of a metal element, for explaining its mechanism elucidated by us. As shown in FIG. 6(A), a buffer layer 12 consisting of silicon oxide and an amorphous silicon film 13 are formed over a glass substrate 11.
Then, a silicon oxide film 14 is formed to a thickness of 500 to 2000 .ANG.. A hole 14a is formed in the film. Typically, the hole 14a takes a rectangular form extending in a direction vertical to the plane of the figure.
In the hole 14a, a thin oxide film (not shown) is formed on the surface of the amorphous silicon film 13 to a thickness of about 10 to 50 .ANG.. This thin oxide film improves the surface characteristics of the amorphous silicon film 13 and thus the amorphous silicon film no longer repels water. The thin oxide film can be formed by ultraviolet radiation within an oxygen ambient or immersing the substrate in ozone water or hydrogen peroxide water.
In order to introduce nickel, which is a metal element for promoting crystallization of silicon, into the amorphous silicon film 13 under this condition, aqueous solution of nickel acetate is applied by spin coating and dried. As a result, an extremely thin nickel film 15 is formed in intimate contact with the surface of the amorphous silicon film 13 in the hole 14a within the silicon oxide film 14.
As shown in FIG. 6(B), the laminate is heat-treated at a temperature of 450 to 640.degree. C. for 4 to 8 hours, typically at 550.degree. C. for 8 hours. Crystals are grown from the regions with which the extremely thin nickel film 15 is in contact in directions parallel to the substrate 11 indicated by the arrows. As a result, a crystalline silicon film 16 is formed. The crystal growth depth can be set to tens of micrometers to 100 .mu.m or more. Where a glass substrate is used as the substrate, the heating temperature is preferably set below the strain point of the glass substrate in order to prevent the glass substrate from shrinking or deforming.
As shown in FIG. 6(C), after the crystal growth, the silicon oxide film 14 is removed. Thereafter, if necessary, laser annealing may be performed to improve the crystallinity of the crystalline silicon film 16. This crystalline silicon film 16 has a region 16a located immediately under the thin nickel film 15. In this region 16a, crystals are grown vertical to the glass substrate 11. The crystallographic axis is not uniform. This growth is referred to as vertical growth. On the other hand, in a region 16b located around the vertical growth region 16a, crystals are grown parallel to the glass substrate 11 with a substantially uniform crystallographic axis. This crystal growth is referred to as lateral growth.
As shown in FIG. 6(D), the crystalline silicon film 16 is patterned to form an active layer 17 for TFTs. A silicon oxide film 18 acting as a gate-insulating film is formed. The TFTs are completed by well-known fabrication techniques. The region located just under the extremely thin nickel film 15 and the regions in which the crystal growth terminates are heavily doped with nickel and so it is necessary that these regions be not contained in the channel formation region.
Silicon crystals can be grown parallel to the substrate 11, i.e., laterally, as shown in FIG. 6(B), by adopting the above-described crystallization techniques. Since the directions of crystals of the obtained crystalline silicon film 16 are uniform, TFTs using this crystalline silicon film 16 show good electrical characteristics and are capable of operating at high speeds.
In the above-described crystallization step, however, after obtaining the crystalline silicon film 16, the mask 14 is removed, and the surface of the crystalline silicon film 16 is exposed. Therefore, there is the possibility that the surface is contaminated. Furthermore, ridges might be formed because the laser annealing is carried out while the surface of the crystalline silicon film 16 is exposed. The contamination and the ridges will raise the energy levels at the interface between the active layer 17 and the gate-insulating film. Hence, the characteristics of the TFTs are deteriorated.